356 A. A. W. HUBRECHT 



the mouth has travelled forwards till close to the tip of the head, 

 the intestinal tract thus passing beyond the proboscidian sheath 

 anteriorly. In certain other Nemertines the proboscidian 

 cavity does not extend through the whole length of the body 

 posteriorly. This is^ for example, the case in that genus which 

 must be regarded for several reasons as the least differentiated, 

 primitive type — the genus Carinella. It is only in the anterior 

 region of the body that the proboscis and the cavity surrounding 

 it are found, the latter situated as usual above the intestine. 

 Here, too, the mouth is found ventrally, the opening for the 

 proboscis terminally. One other genus — Drepanophorus — 

 deserves special mention, in so far as the proboscidian sheath 

 has the bulk of its cavity increased by lateral thin-walled 

 sacs, metamerically placed, one above each lateral caecum 

 of the intestine, and communicating with the cavity of the 

 sheath by narrow perforations of the muscular tissue of its 

 wall. Nemertes carcinophila is said both by M'Intosh 

 and Barrois to be without a special proboscidian sheath. 

 Barrois found the proboscis much reduced (according to him as 

 an effect of parasitism) and floating in the general body-cavity. 

 Not having examined this species myself, and not having 

 either ever met with a general body-cavity in other Nemer- 

 tines, I would venture to suggest the necessity of a careful re- 

 examination of this species, which might prove to be not 

 without importance for the problem we are considering. 



The type according to which the proboscidian sheath is built 

 up is very similar throughout the whole group, although the 

 muscular elements in its wall may increase in number (fig, 16) 

 and become more co-aiplicately arranged, or its size may be consi- 

 derably reduced. It is capable of a very considerable increase 

 in width corresponding to the movements, the rapid retraction, 

 or the mode of coiling up of the proboscis it encloses. It is, 

 moreover, filled with a fluid, containing corpuscles characteristic 

 in shape, and in one case — Cerebratulus urticans — charac- 

 teristic in chemical properties, viz. by the presence of haemo- 

 globin. This fluid is in no way connected with the fluid 

 circulating in the longitudinal and transverse blood-vessels. 



